I would have to say that it is probably the endorphins one gets from riding. I find them fairly addicting till I get to the point where I have overtrained and am a bit sore. It's very similar to what I have heard described as the "runners high". I also get some motivation from posting the latest ride to my riders log spreadsheet and comparing it to last years totals mileage and average speeds.
I also do some riding just for leisure and there it is the ability to get to and from cultural activities without hassling traffic and parking problems. Since I live near the intersection of a number of bike paths, I can get to a lot of events without spending much time on public streets.
Now is a good time of year to get back into riding. I would advise riding often if not necessarily very long to start off with. Find a low hassle route that you can do near your home that doesn't take to long and ride it more days than not. Sooner or later you will probably find yourself at the end of the ride thinking that I could do more of that. Go from there!
